Reporting in this week's journal Nature, an international team of scientists including Helen Amanda Fricker of Scripps Institution of Oceanography at UC San Diego has established that warm ocean currents are the dominant cause of recent ice loss in Antarctica. New measurement techniques have been used to differentiate, for the first time, between the two causes of thinning ice shelves - warm ocean currents melting the underside, and warm air melting from above. This finding brings scientists a step closer to providing reliable projections of future sea-level rise. The work was initiated during a late 2009 visit to Scripps by British Antarctic Survey (BAS) scientist Hamish Pritchard, lead author of the study.

Working with Fricker, Pritchard used measurements made by a laser instrument mounted on NASA's ICESat (Ice, Cloud and Land Elevation Satellite) to estimate the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt around the continent. Of the 54 ice shelves studied, warm ocean currents are melting 20, most of which are in West Antarctica. In every case, the inland glaciers that flow down to the coast and feed into these thinning ice shelves are also draining more ice into the sea, contributing to sea-level rise. Only Larsen Ice Shelf, on the eastern side of the Antarctic Peninsula (the long stretch of land pointing towards South America), is thinning because of warm air above it instead of melting from ocean currents.

"In most places in Antarctica, we can't explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below" said Pritchard.

Ice shelves are where the ice sheet meets the ocean, and basal melting from ice shelves in itself is not unusual - it is a standard 'background' process for the Antarctic ice sheet to lose some of the mass it gains through snowfall so that it remains more or less in balance. The other way ice shelves lose mass is by iceberg calving. This study has confirmed that, under some ice shelves, basal melting is large enough that they are currently thinning, sometimes up to several meters per year.

The study has also shown that when an ice shelf is thinning, the inland glaciers feeding it are also losing ice into the ocean, which will contribute to sea-level rise.

"We've looked all around the Antarctic coast and we see a clear pattern: in all the cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It's this glacier acceleration that's responsible for most of the increase in mass loss from the continent and this is contributing to sea-level rise" said Pritchard.

"In the last decade or so we have seen some inland glaciers accelerate and thin dramatically when an ice shelf completely disappears," said Fricker, an associate professor in Scripps' Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "In this study we also see that glaciers are losing mass to the ocean even while the ice shelves are still there, but getting thinner. This suggests that ice loss is very sensitive to quite small changes in the ocean circulation around Antarctica."

"What's really interesting is just how sensitive these glaciers seem to be," added Pritchard. "Some ice shelves are thinning by a few metres a year and, in response, the glaciers drain billions of tons of ice into the sea. This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet. It means that we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt - the oceans can do all the work from below."

Other recent studies have shown that the rate of ice-shelf thinning is sensitive to quite small changes in the flow of warm ocean currents around Antarctica. Circulation changes are mostly driven by changes in the pattern of winds around Antarctica which, in turn, are related to global changes in climate. The new research shows that understanding how the ocean responds to climate change is critical to forecasting future Antarctic ice loss.

David Vaughan, a co-author of the paper and the leader of ice2sea - a major EU-funded program, summed up the research: "This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance. But the real significance is that it also shows the key to predicting how the ice sheet will change in the future is in understanding the oceans."

The study was carried out by an international team from British Antarctic Survey, Utrecht University, Scripps Institution of Oceanography and Earth & Space Research. NASA's ICESat - Ice, Cloud and Land Elevation Satellite - collected measurements during the period 2003 - 2009.

British Antarctic Survey (BAS), a component of the Natural Environment Research Council, delivers and enables world-leading interdisciplinary research in the Polar Regions. Its skilled science and support staff based in Cambridge, Antarctica and the Arctic, work together to deliver research that uses the Polar Regions to advance our understanding of Earth as a sustainable planet. Through its extensive logistic capability and know-how BAS facilitates access for the British and international science community to the UK polar research operation. Numerous national and international collaborations, combined with an excellent infrastructure help sustain a world leading position for the UK in Antarctic affairs. For more information visit www.antarctica.ac.uk

Ice2sea brings together the EU's scientific and operational expertise from 24 leading institutions across Europe and beyond. Improved projections of the contribution of ice to sea-level rise produced by this major European-funded programme will inform the fifth IPCC report (due in 2013). In 2007, the fourth Intergovernmental Panel on Climate Change (IPCC) report highlighted ice-sheets as the most significant remaining uncertainty in projections of sea-level rise. Understanding about the crucial ice-sheet effects was "too limited to assess their likelihood or provide a best estimate of an upper bound for sea-level rise." Ice2sea is funded by the European Union's Seventh Framework Programme (FP7). www.ice2sea.eu